Electroless metallization of dielectrics with alkaline stable pyrimidine derivative containing catalysts

ABSTRACT

Pyrimidine derivatives which contain one or more electron donating groups on the ring are used as catalytic metal complexing agents in aqueous alkaline environments to catalyze electroless metal plating on metal clad and un-clad substrates. The catalysts are monomers and free of tin and antioxidants.

FIELD OF THE INVENTION

The present invention is directed to electroless metallization ofdielectrics with alkaline stable monomeric pyrimidine derivativecontaining catalysts. More specifically, the present invention isdirected to metallization of dielectrics with alkaline stable monomericpyrimidine derivative containing catalysts as a replacement forpalladium/tin colloidal catalysts.

BACKGROUND OF THE INVENTION

Conventional printed circuit boards (PCBs) consist of laminatednon-conductive dielectric substrates that rely on drilled and platedthrough holes (PTHs) to form a connection between the opposite sidesand/or inner layers of a board. Electroless plating is a well-knownprocess for preparing metallic coatings on surfaces. Electroless platingof a dielectric surface requires the prior deposition of a catalyst. Themost commonly used method to catalyze or activate laminatednon-conductive dielectric substrate regions, prior to electrolessplating, is to treat the board with an aqueous tin-palladium colloid inan acidic chloride medium. The colloid consists of a metallic palladiumcore surrounded by a stabilizing layer of tin(II) ions. A shell of[SnCl₃]⁻ complexes act as surface stabilizing groups to avoidagglomeration of colloids in suspension.

In the activation process, the palladium-based colloid is adsorbed ontoan insulating substrate such as epoxy or polyimide to activateelectroless copper deposition. Theoretically, for electroless metaldeposition, the catalyst particles play roles as carriers in the path oftransfer of electrons from reducing agent to metal ions in the platingbath. Although the performance of an electroless copper process isinfluenced by many factors such as composition of the depositionsolution and choice of ligand, the activation step is the key factor forcontrolling the rate and mechanism of electroless deposition.Palladium/tin colloid has been commercially used as an activator forelectroless metal deposition for decades, and its structure has beenextensively studied. Yet, its sensitivity to air and high cost leaveroom for improvement or substitution.

While the colloidal palladium catalyst has given good service, it hasmany shortcomings which are becoming more and more pronounced as thequality of manufactured printed circuit boards increases. In recentyears, along with the reduction in sizes and an increase in performanceof electronic devices, the packaging density of electronic circuits hasbecome higher and subsequently required to be defect free afterelectroless plating. As a result of greater demands on reliabilityalternative catalyst compositions are required. The stability of thecolloidal palladium catalyst is also a concern. As mentioned above, thepalladium/tin colloid is stabilized by a layer of tin(II) ions and itscounter-ions can prevent palladium from aggregating. The tin(II) ionseasily oxidizes to tin(IV) and thus the colloid cannot maintain itscolloidal structure. This oxidation is promoted by increases intemperature and agitation. If the tin(II) concentration is allowed tofall close to zero, then palladium particles can grow in size,agglomerate, and precipitate.

Considerable efforts have been made to find new and better catalysts.For example, because of the high cost of palladium, much of the efforthas been directed toward the development of a non-palladium orbimetallic alternative catalysts. In the past, problems have includedthe fact that they are not sufficiently active or reliable enough forthrough-hole plating. Furthermore, these catalysts typically becomeprogressively less active upon standing, and this change in activityrenders such catalysts unreliable and impractical for commercial use.U.S. Pat. No. 4,248,632 discloses a non-palladium/tin catalyst forelectroless plating. The catalyst includes a complex of a catalyticmetal, such as palladium or alternative metals such as silver and gold,nitrogen containing ligands and acid radicals; however, an acidenvironment is critical for its activating performance. Acidenvironments typically cause undesired corrosion of metal cladding foundon many dielectric substrates resulting in defective articles. Thisproblem is very common in the manufacture of printed circuit boardswhere the boards are often heavily clad with copper, thus acidenvironments are highly undesirable in the industry.

Preferably metal clad dielectrics are electrolessly plated in alkalineenvironments but many non-palladium/tin catalysts are unstable andunreliable under such conditions. U.S. Pat. No. 5,503,877 disclosesanother non-palladium/tin catalyst which may be used in an acid as wellas an alkaline environment. The catalyst is composed of a catalyticmetal such as palladium, silver or gold, nitrogen containing ligands andsolvent component; however, the catalyst must first be heated forprolonged periods of time prior to use to form an oligomer/polymerotherwise it is insufficiently active. Further, prolonged heating andsubsequent cooling causes more cost due to increased manpower andfacility charges in large scale catalyst preparation. Accordingly, thereis still a need for a replacement catalyst for palladium/tin.

SUMMARY OF THE INVENTION

Methods include providing a substrate including a dielectric; applyingan aqueous alkaline catalyst solution to the substrate including thedielectric, the aqueous alkaline catalyst includes a monomeric complexof metal ions and one or more pyrimidine derivatives having formula:

where R₁, R₂, R₃ and R₄ may be the same or different and are hydrogen,(C₁-C₃)alky, —N(R)₂, hydroxyl, hydroxy(C₁-C₃)alkyl, (C₁-C₃)alkoxy,carboxy or halogen, and where R may be the same or different and ishydrogen or (C₁-C₃)alkyl, and with the proviso that when R₂ and R₄ arehydroxyl, R₁ is also hydroxyl, R₁, R₂ and R₄ cannot be —N(R)₂ at thesame instance and when R₂ is an alkyl, R₁, R₃ and R₄ cannot all behydrogen, and R₁, R₂, R₃ and R₄ are not hydrogen at the same instance;or salts thereof; applying a reducing agent to the substrate includingthe dielectric; and immersing the substrate including the dielectricinto an alkaline metal plating bath to electrolessly plate metal on thesubstrate with the dielectric.

The aqueous alkaline catalysts may be used to electrolessly plate metalson substrates of dielectric materials and substrates which also includemetal cladding. The aqueous alkaline catalysts are storage stable andare stable during electroless metal plating even in alkaline electrolessmetal plating environments. They do not readily oxidize as compared toconventional tin/palladium catalysts even though the aqueous alkalinecatalysts are free of antioxidants. They do not require strong acids toprepare or maintain stability, thus they are less corrosive thanconventional catalysts. They do not require tin compounds for stabilityand may be halogen free. Also, oligomeric/polymeric complex formationwith prolonged heating is not required to form stable and catalyticallyactive metal ligand complexes, thus providing a more efficientelectroless plating method. The catalysts enable good metal coverageduring via and through-hole filling in the manufacture of printedcircuit boards.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of the backlight performance of apalladium/6-hydroxy-2,4-dimethylpyrimidine catalyst vs. a conventionaltin/palladium colloidal catalyst on through-hole walls of multiplesubstrates.

FIG. 2 is a plot of the backlight performance of apalladium/2-amino-4,6-dimethylpyrimidine catalyst vs. a conventionaltin/palladium catalyst on through-hole walls of multiple substrates.

DETAILED DESCRIPTION OF THE INVENTION

As used throughout this specification, the abbreviations given belowhave the following meanings, unless the context clearly indicatesotherwise: g=gram; mg=milligram; mL=milliliter; L=liter; cm=centimeter;m=meter; mm=millimeter; μm=micron; ppm=parts per million; M=molar; °C.=degrees Centigrade; g/L=grams per liter; DI=deionized; Pd=palladium;wt %=percent by weight; and T_(g)=glass transition temperature.

The term “monomer” or “monomeric” means a single molecule which maycombine with one or more of the same or similar molecules. The term“oligomer” means two or three monomers combined to form a singlemolecule. The term “polymer” means two or more monomers combined or twoor more oligomers combined to form a single molecule. The term “halogen”means chlorine, bromine, fluorine and iodine. The terms “printed circuitboard” and “printed wiring board” are used interchangeably throughoutthis specification. The terms “plating” and “deposition” are usedinterchangeably throughout this specification. All amounts are percentby weight, unless otherwise noted. All numerical ranges are inclusiveand combinable in any order except where it is logical that suchnumerical ranges are constrained to add up to 100%.

Aqueous alkaline catalyst solutions include complexes of metal ionschosen from silver, gold, platinum, palladium, copper, cobalt andnickel, and one or more pyrimidine derivative complexing compoundshaving formula:

where R₁, R₂, R₃ and R₄ may be the same or different and are hydrogen,(C₁-C₃)alky, —N(R)₂, hydroxyl, hydroxy(C₁-C₃)alkyl, (C₁-C₃)alkoxy,carboxy or halogen, and where R may be the same or different and ishydrogen or (C₁-C₃)alkyl, and with the proviso that when R₂ and R₄ arehydroxyl, R₁ is also hydroxyl, R₁, R₂ and R₄ cannot be —N(R)₂ at thesame instance and when R₂ is an alkyl, R₁, R₃ and R₄ cannot all behydrogen, and R₁, R₂, R₃ and R₄ are not all hydrogen at the sameinstance; or salts thereof. Preferably R₁, R₂, R₃ and R₄ areindependently hydrogen, (C₁-C₂)alkyl, —N(R)₂ where R may be the same ordifferent and is hydrogen or (C₁-C₂)alkyl; hydroxyl, (C₁-C₂)alkoxy,carboxy or chlorine with the proviso that when R₂ and R₄ are hydroxyl,R₁ is also hydroxyl, R₁, R₃ and R₄ cannot be —N(R)₂ at the same instanceand when R₂ is an alkyl, R₁, R₃ and R₄ cannot all be hydrogen, and R₁,R₂, R₃ and R₄ are not hydrogen at the same instance. More preferably,R₁, R₂, R₃ and R₄ are independently hydrogen, methyl, —NH₂, hydroxyl,methoxy, carboxy or chlorine with the proviso that when R₂ and R₄ arehydroxyl, R₁ is also hydroxyl, R₁, R₃ and R₄ cannot be —NH₂ at the sameinstance and when R₂ is methyl, R₁, R₃ and R₄ cannot all be hydrogen,and R₁, R₂, R₃ and R₄ are not hydrogen at the same instance. Mostpreferably, R₁, R₂, R₃ and R₄ are independently hydrogen, methyl, —NH₂,or hydroxyl, with the proviso that when R₂ and R₄ are hydroxyl, R₁ isalso hydroxyl, R₁, R₃ and R₄ cannot be —NH₂ at the same instance andwhen R₂ is methyl, R₁, R₃ and R₄ cannot all be hydrogen, and R₁, R₂, R₃and R₄ are not hydrogen at the same instance. In general such pyrimidinederivatives are included in the catalysts in amounts of 10 ppm to 500ppm, typically from 60 ppm to 300 ppm.

Examples of such pyrimidine derivatives are uracil, barbituric acid,orotic acid, thymine, 2-aminopyrimidine,6-hydroxy-2,4-dimethylpyrimidine, 6-methyluracil, 2-hydroxypyrimidine,4,6-dichloropyrimidine, 2,4-dimethoxypyrimidine,2-amino-4,6-dimethylpyrimidine, 2-hydroxy-4,6-dimethylpyrimidine and6-methylisocytosine.

Sources of metal ions include any of the conventional water solublemetal salts known in the art and literature which provide metals havingcatalytic activity. One type of catalytic metal ion may be used ormixtures of two or more catalytic metal ions may be used. Such salts areincluded to provide metal ions in amounts of 20 ppm to 350 ppm,preferably from 25 ppm to 250 ppm. Silver salts include, but are notlimited to silver nitrate, silver acetate, silver trifluoroacetate,silver tosylate, silver triflate, silver fluoride, silver oxide, silversodium thiosulfate and silver potassium cyanide. Palladium saltsinclude, but are not limited to palladium chloride, palladium acetate,palladium potassium chloride, palladium sodium chloride, sodiumtetrachloropalladate and palladium nitrate. Gold salts include, but arenot limited to gold cyanide, gold trichloride, gold tribromide,potassium gold chloride, potassium gold cyanide, sodium gold chlorideand sodium gold cyanide. Platinum salts include, but are not limited toplatinum chloride and platinum sulfate. Copper salts include, but arenot limited to copper sulfate and copper chloride. Nickel salts include,but are not limited to nickel chloride and nickel sulfate. Cobalt saltsinclude, but are not limited to cobalt acetate, cobalt chloride, cobaltbromide and cobalt ammonium sulfate. Preferably the metal ions aresilver, palladium and gold ions. More preferably the metal ions aresilver and palladium. Most preferably the ions are palladium.

The components which make up the aqueous alkaline catalysts may becombined in any order. Any suitable method known in the art andliterature may be used to prepare the aqueous catalysts; however, noheating is applied to form an oligomer or polymer of the complexingpyrimidine derivative and the metal ions. The aqueous alkaline catalystsolution is substantially a solution of monomers of pyrimidinederivative complexing compound and metal ions. The amount of pyrimidinederivative complexing compounds and one or more metal ions included inthe aqueous alkaline catalyst solutions are such that a molar ratio ofcomplexing compounds to metal ions is from 1:1 to 4:1, preferably from1:1 to 2:1. In general, one or more of the complexing compounds is firstsolubilized in a sufficient amount of water. One or more sources ofmetal ions are dissolved in a minimal amount of water and then combinedwith the complexing solution with stirring to form a uniform aqueoussolution. Typically the catalyst solution is prepared at roomtemperature but some heating may be required to expedite solubilizationof the components. The pH of the aqueous catalyst solution is adjustedto an alkaline pH with salts such as sodium tetraborate, sodiumcarbonate or alkali metal hydroxides such as potassium or sodiumhydroxide or mixtures thereof. The pH range of the aqueous alkalinecatalyst solution is from 8.5 and greater, preferably from 9 andgreater, more preferably from 9 to 13, most preferably from 9 to 12. Theaqueous alkaline catalysts are free of tin, tin ions and antioxidants.Preferably the aqueous alkaline catalysts are halogen free.

Following application of the catalyst to the substrate and prior tometallization one or more reducing agents are applied to the catalyzedsubstrate to reduce the metal ions to their metallic state. Conventionalreducing agents known to reduce metal ions to metal may be used. Suchreducing agents include, but are not limited to dimethylamine borane,sodium borohydride, ascorbic acid, iso-ascorbic acid, sodiumhypophosphite, hydrazine hydrate, formic acid and formaldehyde.Preferably the reducing agent is sodium hypophosphite. Reducing agentsare included in amounts to reduce substantially all of the metal ions tometal. Such amounts are generally conventional amounts and are wellknown by those of skill in the art.

The aqueous alkaline catalysts may be used to electrolessly metal platevarious substrates such as semiconductors, metal-clad and uncladsubstrates such as printed circuit boards. Such metal-clad and uncladprinted circuit boards may include thermosetting resins, thermoplasticresins and combinations thereof, including fiber, such as fiberglass,and impregnated embodiments of the foregoing. Preferably the substrateis a metal-clad printed circuit or wiring board.

Thermoplastic resins include, but are not limited to acetal resins,acrylics, such as methyl acrylate, cellulosic resins, such as ethylacetate, cellulose propionate, cellulose acetate butyrate and cellulosenitrate, polyethers, nylon, polyethylene, polystyrene, styrene blends,such as acrylonitrile styrene and copolymers and acrylonitrile-butadienestyrene copolymers, polycarbonates, polychlorotrifluoroethylene, andvinylpolymers and copolymers, such as vinyl acetate, vinyl alcohol,vinyl butyral, vinyl chloride, vinyl chloride-acetate copolymer,vinylidene chloride and vinyl formal.

Thermosetting resins include, but are not limited to allyl phthalate,furane, melamine-formaldehyde, phenol-formaldehyde and phenol-furfuralcopolymers, alone or compounded with butadiene acrylonitrile copolymersor acrylonitrile-butadiene-styrene copolymers, polyacrylic esters,silicones, urea formaldehydes, epoxy resins, allyl resins, glycerylphthalates and polyesters.

The catalysts may be used to plate substrates with both low and highT_(g) resins. Low T_(g) resins have a T_(g) below 160° C. and high T_(g)resins have a T_(g) of 160° C. and above. Typically high T_(g) resinshave a T_(g) of 160° C. to 280° C. or such as from 170° C. to 240° C.High T_(g) polymer resins include, but are not limited to,polytetrafluoroethylene (PTFE) and polytetrafluoroethylene blends. Suchblends include, for example, PTFE with polypheneylene oxides and cyanateesters. Other classes of polymer resins which include resins with a highTg include, but are not limited to, epoxy resins, such as difunctionaland multifunctional epoxy resins, bimaleimide/triazine and epoxy resins(BT epoxy), epoxy/polyphenylene oxide resins, acrylonitrilebutadienestyrene, polycarbonates (PC), polyphenylene oxides (PPO),polypheneylene ethers (PPE), polyphenylene sulfides (PPS), polysulfones(PS), polyamides, polyesters such as polyethyleneterephthalate (PET) andpolybutyleneterephthalate (PBT), polyetherketones (PEEK), liquid crystalpolymers, polyurethanes, polyetherimides, epoxies and compositesthereof.

The catalysts may be used to deposit metals on dielectric materials andthe walls of through-holes or vias of printed circuit boards. Thecatalysts may be used in both horizontal and vertical processes ofmanufacturing printed circuit boards.

The aqueous catalysts may be used with conventional alkaline electrolessmetal plating baths. While it is envisioned that the catalysts may beused to electrolessly deposit any metal which may be electrolesslyplated, preferably, the metal is chosen from copper, copper alloys,nickel or nickel alloys. More preferably the metal is chosen from copperand copper alloys, most preferably copper is the metal. An example of acommercially available electroless copper plating bath is CIRCUPOSIT™880 Electroless Copper bath (available from Dow Advanced Materials,Marlborough, Mass.).

Typically sources of copper ions include, but are not limited to watersoluble halides, nitrates, acetates, sulfates and other organic andinorganic salts of copper. Mixtures of one or more of such copper saltsmay be used to provide copper ions. Examples include copper sulfate,such as copper sulfate pentahydrate, copper chloride, copper nitrate,copper hydroxide and copper sulfamate. Conventional amounts of coppersalts may be used in the compositions. In general copper ionconcentrations in the composition may range from 0.5 g/L to 30 g/L.

One or more alloying metals also may be included in the electrolesscompositions. Such alloying metals include, but are not limited tonickel and tin. Examples of copper alloys include copper/nickel andcopper/tin. Typically the copper alloy is copper/nickel.

Sources of nickel ions for nickel and nickel alloy electroless baths mayinclude one or more conventional water soluble salts of nickel. Sourcesof nickel ions include, but are not limited to, nickel sulfates andnickel halides. Sources of nickel ions may be included in theelectroless alloying compositions in conventional amounts. Typicallysources of nickel ions are included in amounts of 0.5 g/L to 10 g/L.

Conventional steps used for electrolessly metal plating a substrate maybe used with the catalysts; however, the aqueous alkaline catalysts donot require an acceleration step where tin is stripped to expose thepalladium metal for electroless plating as in many conventionalprocesses. Accordingly, acceleration steps are excluded when using thecatalysts. Preferably, the catalysts are applied to the surface of thesubstrate to be electrolessly plated with a metal followed byapplication of a reducing agent to the catalyzed substrate and thenapplication of the metal plating bath. Electroless metal platingparameters, such as temperature and time may be conventional. The pH ofthe electroless metal plating bath is alkaline. Conventional substratepreparation methods, such as cleaning or degreasing the substratesurface, roughening or micro-roughening the surface, etching ormicro-etching the surface, solvent swell applications, desmearingthrough-holes and various rinse and anti-tarnish treatments may be used.Such methods and formulations are well known in the art and disclosed inthe literature.

Preferably, the substrate to be metal plated is a metal-clad substratewith dielectric material and a plurality of through-holes such as aprinted circuit board. The boards are rinsed with water and cleaned anddegreased followed by desmearing the through-hole walls. Typicallyprepping or softening the dielectric or desmearing of the through-holesbegins with application of a solvent swell.

Any conventional solvent swell may be used. The specific type may varydepending on the type of dielectric material. Examples of dielectricsare disclosed above. Minor experimentation may be done to determinewhich solvent swell is suitable for a particular dielectric material.The T_(g) of the dielectric often determines the type of solvent swellto be used. Solvent swells include, but are not limited to glycol ethersand their associated ether acetates. Conventional amounts of glycolethers and their associated ether acetates may be used. Examples ofcommercially available solvent swells are CIRCUPOSIT™ Conditioner 3302A,CIRCUPOSIT™ Hole Prep 3303 and CIRCUPOSIT™ Hole Prep 4120 solutions(available from Dow Advanced Materials).

After the solvent swell, a promoter may be applied. Conventionalpromoters may be used. Such promoters include sulfuric acid, chromicacid, alkaline permanganate or plasma etching. Typically alkalinepermanganate is used as the promoter. Examples of commercially availablepromoters are CIRCUPOSIT™ Promoter 4130 and CIRCUPOSIT™ MLB Promoter3308 solutions (available from Dow Advanced Materials). Optionally, thesubstrate and through-holes are rinsed with water.

A neutralizer is then applied to neutralize any residues left by thepromoter. Conventional neutralizers may be used. Typically theneutralizer is an aqueous acidic solution containing one or more aminesor a solution of 3 wt % hydrogen peroxide and 3 wt % sulfuric acid. Anexample of a commercially available neutralizer is CIRCUPOSIT™ MLBNeutralizer 216-5. Optionally, the substrate and through-holes arerinsed with water and then dried.

After neutralizing an acid or alkaline conditioner is applied.Conventional conditioners may be used. Such conditioners may include oneor more cationic surfactants, non-ionic surfactants, complexing agentsand pH adjusters or buffers. Examples of commercially available acidconditioners are CIRCUPOSIT™ Conditioners 3320A and 3327 solutions(available from Dow Advanced Materials). Suitable alkaline conditionersinclude, but are not limited to aqueous alkaline surfactant solutionscontaining one or more quaternary amines and polyamines Examples ofcommercially available alkaline surfactants are CIRCUPOSIT™ Conditioner231, 3325, 813 and 860 formulations. Optionally, the substrate andthrough-holes are rinsed with water.

Conditioning may be followed by micro-etching. Conventionalmicro-etching compositions may be used. Micro-etching is designed toprovide a micro-roughened metal surface on exposed metal (e.g.innerlayers and surface etch) to enhance subsequent adhesion of platedelectroless metal and later electroplate. Micro-etches include, but arenot limited to 60 g/L to 120 g/L sodium persulfate or sodium orpotassium oxymonopersulfate and sulfuric acid (2%) mixture, or genericsulfuric acid/hydrogen peroxide. Examples of commercially availablemicro-etching compositions are CIRCUPOSIT™ Microetch 3330 Etch solutionand PREPOSIT™ 748 Etch solution both available from Dow AdvancedMaterials. Optionally, the substrate is rinsed with water.

Optionally, a pre-dip may then applied to the micro-etched substrate andthrough-holes. Examples of pre-dips include organic salts such as sodiumpotassium tartrate or sodium citrate, 0.5% to 3% sulfuric acid or anacidic solution of 25 g/L to 75 g/L sodium chloride.

The aqueous alkaline catalyst is then applied to the substrate.Application may be done by conventional methods used in the art, such asimmersing the substrate in a solution of the catalyst or by spraying orby atomization using conventional apparatus. Catalyst dwell time mayrange from 1 minute to 10 minutes, typically from 2 minutes to 8 minutesfor vertical equipment and for 25 seconds to 120 seconds for horizontalequipment. The catalysts may be applied at temperatures from roomtemperature to 80° C., typically from 30° C. to 60° C. The substrate andthrough-holes optionally may be rinsed with water after application ofthe catalyst.

The reducing solution is then applied to the substrate to reduce themetal ions of the catalyst to their metallic state. The reducingsolution may be applied by immersing the substrate into the reducingsolution, spraying the reducing solution onto the substrate or byapplying the solution by atomization. The temperature of the solutionmay range from room temperature to 65° C., typically from 30° C. to 55°C. Contact time between the reducing solution and the catalyzedsubstrate may range from 30 seconds to 5 minutes before application ofthe electroless metal plating bath. Typically the reducing solution is 6and higher.

The substrate and walls of the through-holes are then electrolesslyplated with metal, such as copper, copper alloy, nickel or nickel alloyusing an electroless bath. Preferably copper is plated on the walls ofthe through-holes. Plating times and temperatures may be conventional.Typically metal deposition is done at temperatures of 20° C. to 80°,more typically from 30° C. to 60° C. The substrate may be immersed inthe electroless plating bath or the electroless bath may be sprayed ontothe substrate. Typically, electroless plating may be done for 5 secondsto 30 minutes; however, plating times may vary depending on thethickness of the metal desired. Plating is done in an alkalineenvironment to prevent undesired corrosion of any metal cladding on thesubstrate. Typically the pH of the plating solution is 8 and higher,preferably the pH is 8.5 and greater, more preferably the pH is from 9to 13.

Optionally anti-tarnish may be applied to the metal. Conventionalanti-tarnish compositions may be used. An example of anti-tarnish isANTI TARNISH™ 7130 solution (available from Dow Advanced Materials). Thesubstrate may optionally be rinsed with water and then the boards may bedried.

Further processing may include conventional processing by photoimagingand further metal deposition on the substrates such as electrolyticmetal deposition of, for example, copper, copper alloys, tin and tinalloys.

The aqueous alkaline catalysts may be used to electrolessly plate metalson substrates of dielectric materials and substrates which also includemetal cladding. The aqueous alkaline catalysts are storage stable andare stable during electroless metal plating even in alkaline electrolessmetal plating environments. They do not readily oxidize as compared toconventional tin/palladium catalysts even though the aqueous alkalinecatalysts are free of antioxidants. They do not require strong acids toprepare or maintain stability, thus they are less corrosive thanconventional catalysts. They do not require tin compounds for stabilityand may be halogen free. Also, oligomeric/polymeric complex formationwith prolonged heating is not required to form stable and catalyticallyactive metal ligand complexes providing a more efficient electrolessplating method. The catalysts enable good metal coverage during via andthrough-hole filling in the manufacture of printed circuit boards.

The following examples are not intended to limit the scope of theinvention but to further illustrate the invention.

Example 1

An aqueous alkaline catalyst solution containing 75 ppm palladium ionsand 85 ppm 6-hydroxy-2,4-dimethylpyrimide (HDMP) in one liter of waterwas prepared by diluting a 17 mL aliquot of a 5 g/L HDMP stock solutionwith 400 mL of DI water. The pH of the solution was adjusted to 10.5with 1 M sodium hydroxide. 188 mg of palladium nitrate hydrate wasdissolved in a minimum of DI water and added to the HDMP solution. 1.9 gof sodium tetraborate decahydrate was dissolved in a one liter beakercontaining 400 mL of DI water and the palladium nitrate and HDMPsolution was added to it. The mixture was then diluted to one liter andstirred 30 minutes at room temperature. The molar ratio of HDMP topalladium ions was 1:1. The pH of the solution was 9.

Example 2

Two each of six different panels with a plurality of through-holes wereprovided: TUC-662, SY-1141, SY-1000-2, IT-158, IT-180 and NPG-150. Thepanels were either four-layer or eight-layer copper-clad panels. TUC-662was obtained from Taiwan Union Technology, and SY-1141 and SY-1000-2were obtained from Shengyi. IT-158 and IT-180 were obtained from ITEQCorp. and NPG-150 was obtained from NanYa. The T_(g) values of thepanels ranged from 140° C. to 180° C. Each panel was 5 cm×12 cm. Thethrough-holes of each panel were treated as follows:

-   -   1. The through-holes of each panel were desmeared with        CIRCUPOSIT™ MLB Conditioner 211 solution for 7 minutes at 80°        C.;    -   2. The through-holes of each panel were then rinsed with flowing        tap water for 4 minutes;    -   3. The through-holes were then treated with CIRCUPOSIT™ MLB        Promoter 3308 aqueous permanganate solution at 80° C. for 10        minutes;    -   4. The through-holes were then rinsed for 4 minutes in flowing        tap water;    -   5. The through-holes were then treated with a 3 wt % sulfuric        acid/3 wt % hydrogen peroxide neutralizer at room temperature        for 2 minutes;    -   6. The through-holes of each panel were then rinsed with flowing        tap water for 4 minutes;    -   7. The through-holes of each panel were then treated with        CIRCUPOSIT™ Conditioner 3325 alkaline solution for 5 minutes at        60° C.;    -   8. The through-holes were then rinsed with flowing tap water for        4 minutes;    -   9. The through-holes were then treated with a sodium        persulfate/sulfuric acid etch solution for 2 minutes at room        temperature;    -   10. The through-holes of each panel were then rinsed with        flowing DI water for 4 minutes;    -   11. Half of the panels were then immersed in CATAPREP™ 404        Pre-Dip solution at room temperature for 1 minute followed by        immersing the panels in a solution of a conventional        palladium/tin catalyst having 75 ppm of palladium metal with an        excess of tin for 5 minutes at 40° C.; while the other half of        the panels were immersed in the palladium ions/HDMP catalyst        prepared in Example 1 for 5 minutes at 40° C.;    -   12. The panels which were treated with the catalyst containing        palladium ions and HDMP were then immersed in a 0.25M solution        of sodium hypophosphite reducing agent at 50° C. for 1 minute to        reduce the palladium ions to palladium metal and then rinsed        with flowing DI water for 30 seconds;    -   13. The panels were then immersed in CIRCUPOSIT™ 880 Electroless        Copper plating bath at 38° C. and at a pH of 13 and copper was        deposited on the walls of the through-holes for 15 minutes;    -   14. The copper plated panels were then rinsed with flowing tap        water for 4 minutes;    -   15. Each copper plated panel was then dried with compressed air;        and    -   16. The walls of the through-holes of the panels were examined        for copper plating coverage using the backlight process        described below.

Each panel was cross-sectioned nearest to the centers of thethrough-holes as possible to expose the copper plated walls. Thecross-sections, no more than 3 mm thick from the center of thethrough-holes, were taken from each panel to determine the through-holewall coverage. The European Backlight Grading Scale was used. Thecross-sections from each panel were placed under a conventional opticalmicroscope of 50× magnification with a light source behind the samples.The quality of the copper deposits was determined by the amount of lightvisible under the microscope that was transmitted through the sample.Transmitted light was only visible in areas of the plated through-holeswhere there was incomplete electroless coverage. If no light wastransmitted and the section appeared completely black, it was rated a 5on the backlight scale indicating complete copper coverage of thethrough-hole wall. If light passed through the entire section withoutany dark areas, this indicated that there was very little to no coppermetal deposition on the walls and the section was rated 0. If sectionshad some dark regions as well as light regions, they were rated between0 and 5. A minimum of ten through-holes were inspected and rated foreach board.

FIG. 1 is a backlight rating distribution graph showing the backlightperformance of both catalysts for each of the six types of panelsplated. The plots in the graph indicate a 95% confidence interval forthe backlight ratings of ten through-holes sectioned for each board. Thehorizontal line through the middle of each plot indicates the averagebacklight value for each group of ten through-hole sections measured.The palladium/HDMP catalyst performed substantially the same as theconventional palladium/tin colloidal catalyst with backlight values ofgreater than 4.5. Backlight values of 4.5 and greater are indicative ofcommercially acceptable catalysts in the plating industry.

Example 3

An aqueous alkaline catalyst solution containing 75 ppm palladium ionsand 105 ppm 2-amino-4,6-dimethylpyrimide (ADMP) in one liter of waterwas prepared by diluting a 21 mL aliquot of a 5 g/L ADMP stock solutionwith 400 mL of DI water. 188 mg of palladium nitrate hydrate wasdissolved in a minimum of DI water and added to the ADMP solution. ThepH of the solution was adjusted to 8.5 with 1M sodium hydroxide. 1.9 gof sodium tetraborate decahydrate was dissolved in a one liter beakercontaining 400 mL of DI water and the palladium nitrate and ADMPsolution was added to it. The mixture was then diluted to one liter andstirred 30 minutes at room temperature. The molar ratio of ADMP topalladium ions was 1.2:1. The pH of the solution was 9.

Example 4

Two each of six different multi-layer, copper-clad panels with aplurality of through-holes were provided as in Example 2: TUC-662,SY-1141, SY-1000-2, IT-158, IT-180 and NPG-150. The through-holes ofeach panel were treated as follows:

-   -   1. The through-holes of each panel were desmeared with        CIRCUPOSIT™ MLB Conditioner 211 solution for 7 minutes at 80°        C.;    -   2. The through-holes of each panel were then rinsed with flowing        tap water for 4 minutes;    -   3. The through-holes were then treated with CIRCUPOSIT™ MLB        Promoter 3308 aqueous permanganate solution at 80° C. for 10        minutes;    -   4. The through-holes were then rinsed for 4 minutes in flowing        tap water;    -   5. The through-holes were then treated with a 3 wt % sulfuric        acid/3 wt % hydrogen peroxide neutralizer at room temperature        for 2 minutes;    -   6. The through-holes of each panel were then rinsed with flowing        tap water for 4 minutes;    -   7. The through-holes of each panel were then treated with        CIRCUPOSIT™ Conditioner 3320A alkaline solution for 5 minutes at        45° C.;    -   8. The through-holes were then rinsed with flowing tap water for        4 minutes;    -   9. The through-holes were then treated with sodium        persulfate/sulfuric acid etch solution for 2 minutes at room        temperature;    -   10. The through-holes of each panel were then rinsed with        flowing DI water for 4 minutes;    -   11. Half of the panels were then immersed in CATAPREP™ 404        Pre-Dip solution at room temperature for 1 minute followed by        immersing the panels in a solution of a conventional        palladium/tin catalyst having 75 ppm of palladium metal with an        excess of tin for 5 minutes at 40° C.; while the other half of        the panels were immersed in the palladium ions/AMDP catalyst        prepared in Example 3 for 5 minutes at 40° C.;    -   12. The panels which were treated with the catalyst containing        palladium ions and AMDP were then immersed in a 0.25M solution        of sodium hypophosphite reducing agent at 50° C. for 1 minute to        reduce the palladium ions to palladium metal and then rinsed        with flowing DI water for 30 seconds;    -   13. The panels were then immersed in CIRCUPOSIT™ 880 Electroless        Copper plating bath at 38° C. and at a pH of 13 and copper was        deposited on the walls of the through-holes for 15 minutes;    -   14. The copper plated panels were then rinsed with flowing tap        water for 4 minutes;    -   15. Each copper plated panel was then dried with compressed air;        and    -   16. The walls of the through-holes of the panels were examined        for copper plating coverage using the backlight process        described below.

Each panel was cross-sectioned nearest to the centers of thethrough-holes as possible to expose the copper plated walls. Thecross-sections, no more than 3 mm thick from the center of thethrough-holes, were taken from each panel to determine the through-holewall coverage. The European Backlight Grading Scale was used asdescribed in Example 2.

FIG. 2 is a backlight rating distribution graph showing the backlightperformance of both catalysts for each of the six types of panelsplated. The plots in the graph indicate a 95% confidence interval forthe backlight ratings of ten through-holes sectioned for each board. Thehorizontal line through the middle of each plot indicates the averagebacklight value for each group of ten through-hole sections measured.The palladium/ADMP catalyst performed better than the conventionalpalladium/tin colloidal catalyst with backlight values of greater than4.5 while the backlight values of the conventional catalyst were 4.5 orjust slightly above 4.5.

Examples 5-17

The pyrimidine derivatives listed in the table below were tested forcatalyst stability and activity on a 40 mL scale. All of the catalystswere prepared with 75 ppm palladium ions and 1.9 g/L sodium tetraboratedecahydrate as a pH buffer to maintain a pH of around 9. With theexception of Examples 9, 13 and 14, two samples of each complexing agentwere tested at different amounts. The molar ratio of palladium ions tocomplexing agent were either 1:1 or 1:2. The molar ratio of palladiumions to complexing agent in Examples 9, 13 and 14 were 1:1.

Stock Solutions:

-   -   a) Complexing agent solutions were 5 g/L;    -   b) Palladium ion solution of 5 g/L was prepared from palladium        nitrate hydrate; and    -   c) Sodium tetraborate decahydrate solution was 25 g/L.

Working Bath Formulations and Method:

-   -   1. A aliquot of the complexing agent was diluted in 30 mL DI        water;    -   2. pH of complexing agent solution was adjusted to around 10.5        with 1M sodium hydroxide for Examples 5-8, 10-14 and 16-17;    -   3. 0.6 mL palladium ion solution was added, the catalysts were        stirred for 5 minutes at room temperature and 1M sodium        hydroxide was added to the 2-aminopyrimidine based complexing        agents of Examples 9 and 15 to adjust the pH to around 8.5; and    -   4. 3.05 mL of borate solution was added and the catalyst was        diluted to 40 mL to adjust the pH to around 9.    -   5. Screen plating activity on small scale (beaker) tests using        un-clad, laminate NY-1140 samples from NanYa was done according        to the following process:        -   a) 10% CIRCUPOSIT™ Conditioner 3325 alkaline solution was            applied to the laminates in Examples 5-8, 10-14 and 16-17            and 7% CIRCUPOSIT™ Conditioner 3320A acid solution was            applied to the laminates of Examples 9 and 15 for 5 minutes            at 50° C. then rinsed with DI water;        -   b) Catalyst baths were applied to the laminates for 5            minutes at 40° C.;        -   c) 0.25 M NaH₂PO₂ reducing agent was applied to the            laminates for 1 minute at 50° C. followed by DI water rinse;        -   d) Laminates were plated with copper using CIRCUPOSIT™ 880            Electroless Copper plating bath for 15 minutes at 40° C.            followed by rinsing with DI water then dried at room            temperature; and        -   e) The laminates were examined for copper plating            performance. The results are shown in Table 1.

TABLE 1 Concen- Exam- tration Copper ple Complexing Agent Stability(ppm) Coverage 5 Uracil yes 80 and 160 Full coverage 6 Barbituric acidyes 90 and 180 Full coverage 7 Orotic acid Partially 120 and 240  Fullcoverage stable: cloudy over- night 8 Thymine Yes 90 and 180 Fullcoverage 9 2-aminopyrimidine Small  80 Full coverage amount of pre-cipitate ob- served after one week at room temper- ature and exposed toair 10 6-hydroxy-2,4- yes 85 and 170 Full coverage dimethylpyrimidine 116-methyluracil yes 90 and 180 Full coverage 12 2- yes 67 and 134 Fullcoverage hydroxypyrimidine hydrochloride salt 13 4,6- yes 105 75%coverage dichloropyrimidine due to skip plating 14 2,4- yes 198 Fullcoverage dimethoxpyrimidine 15 2-amino-4,6- yes 87 and 174 Full coveragedimethylpyrimidine 16 2-hydroxy-4,6- yes 88 and 175 Full coveragedimethylpyrimidine 17 6- yes 88 and 175 Full coverage methylisocytosine

All of the laminates had bright copper deposits with full uniformcoverage with the exception of Example 13 where the complexing agent was4,6-dichloropyrimidine; however, the section of the laminate which wascovered was bright.

In addition to the appearance of the copper deposits, the stability ofthe complexing agent solutions were also observed over a 2-4 hour periodat 40° C. during preparation of the laminates and copper plating andthen overnight after plating when the solutions were cooled to roomtemperature. Most of the test samples were stable during heating andovernight. With the exceptions of Examples 7 and 9, no precipitation orcloudiness was observed. Nevertheless, both orotic acid and2-aminopyrimidine provided full copper coverage on the laminates.

Comparative Examples 1-4

The stability tests and electroless copper plating on laminate NY-1140samples were repeated except that the complexing agents used were thosein Table 2 below. The pyrimidine derivatives had hydroxyl substitutionsin the 4 and 6 positions of the pyrimidine ring without a substitutionat the 2 position, or hydroxyl substitutions at the 4 and 6 positionsand an amine group at the number 2 position, or amine substitutions atpositions 2, 4 and 6 of the pyrimidine ring. The solutions were preparedin the same way as described in Examples 5-17.

TABLE 2 Compar- ative Concen- Exam- tration Copper ple Complexing AgentStability (ppm) Coverage 1 2-amino-4,6- yes 90 and 180 nodihydroxypyrimidine 2 2,4,6- no 45, 88 and 175 — triaminopyrimidine 34-methylpyrimidine no 65 and 130 — 4 4,6- yes 80 and 160 nodihydroxypyrimidine

Although the catalysts of Comparative Examples 1 and 4 were stable, theyfailed to catalyze electroless copper plating on the laminates. Thecatalysts of Comparative Examples 2 and 3 were unstable. Precipitationwas observed within minutes of mixing the complexing agent and palladiumnitrate hydrate.

What is claimed is:
 1. A method comprising: a) providing a substratecomprising a dielectric; b) applying an aqueous alkaline catalystsolution to the substrate comprising the dielectric, the aqueousalkaline catalyst comprises a monomeric complex of metal ions and one ormore pyrimidine derivatives having formula:

wherein R₁, R₂, R₃ and R₄ may be the same or different and are hydrogen,(C₁-C₃)alky, —N(R)₂, hydroxyl, hydroxy(C₁-C₃)alkyl, (C₁-C₃)alkoxy,carboxy or halogen, and where R may be the same or different and ishydrogen or (C₁-C₃)alkyl, and with the proviso that when R₂ and R₄ arehydroxyl, R₁ is also hydroxyl, R₁, R₂ and R₄ cannot be —N(R)₂ at thesame instance and when R₂ is an alkyl, R₁, R₃ and R₄ cannot all behydrogen, and R₁, R₂, R₃ and R₄ are not hydrogen at the same instance;or salts thereof; c) applying a reducing agent to the substratecomprising the dielectric; and d) immersing the substrate comprising thedielectric into an alkaline metal plating bath to electrolessly platemetal on the substrate comprising the dielectric.
 2. The method of claim1, wherein the one or more pyrimidine derivatives are chosen fromuracil, barbituric acid, orotic acid, thymine, 2-aminopyrimidine,6-hydroxy-2,4,6-triaminopyrimidine, 6-methyluracil, 2-hydroxypyrimidine,4,6-dichloropyrimidine, 2,4-dimethoxypyrimidine,2-amino-4,6-dimethylpyrimidine, 2-hydroxy-4,6-dimethylpyrimidine and6-methylisocytosine.
 3. The method of claim 1, wherein a molar ratio ofthe one or more pyrimidine derivatives to the metal ions is 1:1 to 4:1.4. The method of claim 1, wherein the metal ions are chosen frompalladium, silver, gold, platinum, copper, nickel and cobalt.
 5. Themethod of claim 1, wherein the metal on the substrate is copper, copperalloy, nickel or nickel alloy.
 6. The method of claim 1, wherein theaqueous alkaline catalyst solution has a pH of 8.5 or greater.
 7. Themethod of claim 6, wherein the aqueous alkaline catalyst solution has pHof 9 or greater.
 8. The method of claim 1, wherein the substratecomprising the dielectric further comprises a plurality ofthrough-holes.
 9. The method of claim 8, wherein the substratecomprising the dielectric further comprises metal cladding.